The present invention relates to a heterojunction bipolar transistor (hereinafter referred to as HBT) capable of rapidly operating. More particularly the present invention relates to a wide band gap HBT.
To make a conventional homojunction transistor operate rapidly, it is necessary to form a thin base region. However a thin base region undesirably causes a punch through. On the other hand, if the concentration of the base region is made high to prevent the punch through, the gain of the transistor is lowered. Thus, there is a limit in realizing both high gain and rapid operation in conventional transistors.
In the view of the above-mentioned circumstances, earnest researches of Si(silicon)-HBT have been made to realize rapid operation in Si bipolar transistors. As Si-HBT which have been studied, there are wide-gap-emitter-type ones, wherein material such as silicon carbide having a wider band gap than that of Si is applied to an emitter, and narrow-gap-base-type ones, wherein material such as mixed crystal of Si and Ge having a narrower band gap than that of Si is applied to a base region.
In a conventional wide band gap HBT, Si semiconductor crystal is applied to collector region and base region, and 3c-SiC (3c-silicon carbide or .beta.-SiC), which has a wider band gap than that of Si, is applied to emitter region so that emitter injection efficiency increases and base resistance decreases. Thus the conventional wide band gap HBT is used as an element for rapid operation and high power. These are disclosed in Japanese Unexamined Patent Publication No. 216364/1987.
However, the conventional wide band gap HBT, wherein the same semiconductor material (Si) as that of base region is applied to collector region and positive holes are injected from the base to the collector, has a problem that a large amount of positive holes accumulate in the collector region due to a comparatively thick collector region so that diode operation becomes tardy.
On the other hand, in a conventional HBT wherein SiGe having a narrow band gap is applied to base region, the injection of positive holes into collector region (n-type) is prevented thanks to the band gap difference, caused by the heterojunction, between the base region and collector region, and the width of baser region can be made small to be about 10 nm to reduce the number of electrons accumulated therein. Because operation speed of transistor depends on an amount of accumulated electrons in base region (p-type), high speed operation can be realized. This is disclosed in "Estimate of effects of heterojunction between base and collector in SiGe based HBT" by Ugami and Amamiya, pages 25 to 30, 1988, SDM88-142, The Journal of the Institute of Electronics, Information and Communication Engineers. However, this type of HBT requires MBE apparatus or the like when the formation of heterojunction is performed so that it leaves problems in productivity, cost or the like.
In view of the above-mentioned problems, an object of the present invention is to provide a heterojunction bipolar transistor, which is easily produced and is capable of rapidly operating.